A low loss quartz resonator typically comprises a quartz plate of suitable crystal orientation or cut driven via electrodes disposed on either side of the plate. It has been found that by thickening the electroded portion of the plate, either by metallising the electrodes or by providing the plate with a convex contour, it is possible to trap the mechanical vibration within the electrode region so as to provide a low loss device. The device can be mounted via its edge or rim without significant interference with the vibration of its active region. Typically the device is mounted at discrete points using spring clips and a silver loaded resin whereby electrical contact to the device is effected.
Such a design has a number of problems. The frequency of the device is inversely proportional to its thickness and is thus sensitive to the presence of surface films such as water or organic materials. It is for this reason that close tolerance devices are hermetically sealed either in vacuum or in a dry nitrogen atmosphere. However, it has been found that the silver loaded region used for contacting the device is a source of trace organic materials which cause ageing of the device.
Typically the trapped vibrational mode of the device is a thickness shear mode. However, such a mode is inevitably associated with flexural vibrations which are not trapped and hence reach the edge of the device. With conventional discrete point mounts, these vibrations are partially reflected back into the resonator area. At certain frequencies these interfere constructively to produce whole-plate resonances. These resonances can be close together in frequency as well as having poor temperature coefficients. The result is that as their frequency passes through the frequency of the wanted trapped resonance, they interfere with that resonance and cause `activity dips` with an associated frequency glitch. Furthermore, slight variations in the boundary conditions at the edge of the plate can cause large changes in the frequencies of these plate resonances, and hence the temperatures at which they interfere with the main mode. This leads to thermal hysteresis which causes problems with temperature compensated crystal oscillators.
Another major problem with conventional discrete point mounting is the vibration sensitivity of the final device. In theory if the mount was completely symmetrical, and the vibration was placed at the centre of symmetry, then the vibration sensitivity vanishes in all three axes. However, in practice this is very difficult to achieve because of the difficulty in placing the silver loaded resin or other mounting structures at the precise point required.
The object of the invention is to minimise or to overcome these disadvantages.
One approach to the above problem is to mount the device in a quartz package. This technique is discussed in our specification No. 2202989B which describes and claims a crystal resonator assembly, including a crystal resonator, and first and second housing members mated together to define a cavity in which the resonator is located, wherein the housing members are formed from the same crystal material and hence the same crystal orientation as the resonator.
Whilst this structure has proved satisfactory in operation, the manufacture of the housing members within which the resonator is encapsulated represents a significant process cost.